If you plan to visit Suzhou, Hangzhou, Yangzhou, and the small canal towns along the way, the dreariness of winter can be very charming and atmospheric. I had the chance to visit West Lake in Hangzhou when it was both foggy and sunny, and I liked the foggy weather much better. Ditto for Yangzhou and the rest of the above mentioned places for that matter.

Correct me if I'm off; barring extreme circumstances, I expect the average temp in Shanghai in January to be 50 degrees F day, 35 degrees F night.